Meridian Energy has lost about 14,000 customers since the start of the year, while Genesis Energy has gained more than 5000.
A March quarter report shows Meridian lost its customers to the independent retailer, Energy Direct, but its online subsidiary, Powershop increased its customer numbers by 3%.
The state-owned power company has about 270,600 customers, including Powershop ones.
Meanwhile, Genesis' quarterly report shows gas and electricity customers increased to just over 669,500.
Corporate affairs general manager Dean Schmidt said that confirms Genesis' position as the country's biggest retailer.